1. Field of the Invention
The present invention relates to a porous polytetrafluoroethylene (hereinafter referred to as “PTFE”) membrane and a method of producing the PTFE membrane, and a filter medium using the porous PTFE membrane.
2. Related Background Art
Conventionally, as filter media for air filters, filters for vacuum cleaners, bag filters for incinerators, membrane filters, and the like, filter media including porous PTFE membranes are used. Such porous PTFE membranes are required to have not only air permeability but also high efficiency in collecting particles contained in a gas. For example, a porous PTFE membrane produced so as to have an average pore diameter of 1 μm or less has been proposed (see, for example, Japanese Patent No. 2792354).
In recent years, however, high air permeability, rather than particle collection efficiency, has been required preferentially for some purposes. For example, in the case where a filter medium for dust protection is provided on a container including gas, the external heat or cool air causes the gas in the container to expand or contract, which generates a pressure difference between the inside and the outside of the container. If the filter medium has a high air permeability, it has a high capability of eliminating the pressure difference between the inside and the outside of the container, and reduces the load on the container accordingly. For this reason, in using a specific container or an apparatus, high air permeability is required for a porous PTFE membrane in order to reduce the load imposed on the container due to a change in the external environment.
JP 08 (1996)-32791 B discloses the following production method for obtaining a porous PTFE membrane having a high air permeability. First, a mixture of a PTFE fine powder and a liquid lubricant is formed into a sheet-form body by an extrusion method, and then this sheet-form body is stretched preliminarily in the width direction. Next, the liquid lubricant is removed from the sheet-form body by heating, and then the sheet-form body is stretched biaxially at a temperature equal to or lower than the melting point (327° C.) of PTFE. Thereafter, heat setting is carried out by heating the stretched sheet-form body to a temperature equal to or higher than the melting point of PTFE.
The production method disclosed in JP 08 (1996)-32791 B, however, requires a large number of steps because of the preliminary stretching and heat setting.